Bike Nutrition - how do you do it?

Could be a combo of things.  If you took in a lot of carbs (as you describe) less than 3 hours before your OWS you may have triggered a hypoglycemic response that doomed your subsequent workout.  Here are some other articles (below) that may help you with determining caloric requirements.  Often, people think you need to take in more calories when in fact the solution is taking in LESS calories per hour.  Good luck.

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I'm training for an IM and working on my nutrition. I've a system that works well for me but am developing plan B and plan C options. Read many IM race reports and you'll often see what normally works fails to work in the IM and they need to try different options.

You probably need to look at your sweat rate and be sure you are hydrating enough as well as taking in enough sodium and electrolyte even more imporatant if you are a heavy sweater or it is hot.

Plan A: 325-350 cal/hour
Gatorade Endurance 1 bottle 150 cal./600 mg sodium; water 1 bottle
Food every 20': Gel on 60'; other food 35-50 calories on 20' & 40'

Plan B:
Gatorade Endurance 1.5- 2bottles an hour which is 225-300 cal/hour
Gel 1 an hour: 110 calories

Plan C:
Water- 1.5-2 bottles an hour
Every 15' Lava salt of 255 mg of sodium each
on 15' and 45' gel
on 30' & 60' food 35-50 calories

35-50 calorie food: Strawberry newton, 1/2 banana, shot blok, candy like gummy bears or no or low fat like 3 Musketeer minix2, chcolate covered pretzels

It is a lot to carry for sure...jersey pockets & bento box helps. With my longer rides I have to stop to refill water so I add more food too.

I tried packing a bunch of groceries with me on my long rides because of bonking issues until a couple friends suggested "Infinit".  Now, I just take a couple GU's with me along with the Infinit.

It's good stuff. 

I think one banana post only is not enough.

I did something very similar to you today:

early morning 2.2km in the pool and no breakfast before

rich brekfast with banana, skim milk and plenty of cereals (I can not say how much but a lot)

1 hour rest on the sofa watching TV

3h30' ride on a hilly course where I did 92km at easy pace and during the ride I had a small fruit bar in the second hour (84kcal) and I finished two bottles of 750ml each. One was plain water and the other one Powerbar orange.

Then I closed with a brick of 10km exactly in 45'40".

As you can see, I prefer to eat well ALWAYS before I do sport, either the night before (as I did yesterday night) or right before the ride.

For me, eating while I do sport is pretty difficult.

I'm the one that does the little debbies. I eat every 30 min and I strive to hit 300-400 calories per hour. (mostly it's around 300 and then every other hour I'll have a coke or something that pumps it up).

I found that on less than 300 calories per hour I felt awful when I got done.

At 300-400, I feel tired but good when I get done, which is important if you have run after riding!

My coach is the one who told me to target 400 per hour. And I am a 5'4" 135 lb female.  FWIW, she does the same thing and is a 5'3" 120 lb female. I think though, around 400 is about as much as your body can absorb while exercising. I know on the run I only get about 200 calories per hour.

I pack as much on me as I can and then take money to stop at convenience stores (little debbies are only $.35!)

Edited to add:

Last year during my HIM, my coach was riding her bike along the run route where I was plugging away, slow but I felt strong and was keeping my pace where I wanted it. We passed by a very fit looking dude who was obviously miserable, walking, who looked about ready to collapse. My coach, within earshot of said fit dude, says "that's what happens when you don't nail your nutrition".

Harsh, but true.